1. Field of the Invention
The present invention relates to a method for producing a positive electrode active substance, and use of said active substance. More particularly, the present invention relates to a method for producing a positive electrode active substance that gives a non-aqueous electrolyte secondary battery excellent in cycle characteristics, the positive electrode active substance, a positive electrode and a non-aqueous electrolyte secondary battery.
2. Description of the Related Art
As a non-aqueous electrolyte secondary battery, a lithium secondary battery is put into practical use and is widely prevalent. Further, in recent years, the lithium secondary battery is attracting people's attention not only as a small one for a portable electronic apparatus but also as a large-capacity device for being mounted on a vehicle or for electric power storage. For this reason, there is an enhanced demand for safety, cost reduction, long lifetime and the like.
The lithium secondary battery has a positive electrode, a negative electrode, an electrolytic solution, a separator and an outer cladding material as principal constituent elements. Also, the above positive electrode is constituted of a positive electrode active substance, an electroconductive material, a collector and a binder (binding agent).
Generally, as the positive electrode active substance, a layered transition metal oxide such as represented by lithium cobaltate (LiCoO2) is used. However, the layered transition metal oxide is liable to provoke oxygen elimination in a fully charged state at a comparatively low temperature around 150° C. The oxygen elimination induces thermal bursting reaction of the battery. Therefore, when the battery having lithium cobaltate is used in the portable electronic apparatus, there is a fear that heat generation, fire catching and the like may occur.
For this reason, a lithium-containing composite oxide, for example, lithium iron phosphate (LiFePO4), having a stable structure that does not release oxygen at an abnormal time and having an olivine structure safer than LiCoO2 is expected as the positive electrode active substance. Since lithium iron phosphate does not contain cobalt which has a low degree of presence in the earth crust, lithium iron phosphate also has an advantage of being comparatively less expensive. Also, lithium iron phosphate has an advantage of being more stable in structure than the layered transition metal oxide.
However, when lithium iron phosphate is used as the positive electrode active substance, decrease in the discharging capacity accompanying a repetition of charging and discharging is large, thereby raising a problem of short lifetime of the obtained battery. Regarding lithium iron phosphate, expansion or contraction of the positive electrode active substance caused by intercalation and deintercalation of Li in charging and discharging is large. For this reason, when the number of cycles increases, the positive electrode active substance physically drops off gradually from a collector or a conductive material. By dropping off, the structure of the positive electrode active substance is destroyed, and an active substance that does not contribute to charging and discharging increases. As a result of this, decrease in the discharging capacity occurs, and the lifetime of the battery will be short. In order to cope with this, a method of restraining the expansion and contraction of the positive electrode active substance by using, as the positive electrode active substance, a lithium-containing composite oxide obtained by the solid phase method and prepared by using lithium iron phosphate as a basic structure and performing element substitution is now studied (for example, Patent Document 1: Japanese Unexamined Patent Publication No. 2002-198050 and Patent Document 2: Japanese Unexamined Patent Publication No. 2005-519451).